Method of fabricating roll-bonded expanded load-bearing aluminum laminate structural elements for vehicle

ABSTRACT

An expanded laminate and method of forming an expanded laminate are disclosed. In at least one embodiment, the method includes selectively applying a relatively high temperature-resistant material to a surface of a first metal sheet to form a covered portion and an uncovered portion and diffusion bonding the first metal sheet to a second metal sheet at the uncovered portion to form a bonded region and an unbonded region. Pressurized gas may be introduced between the first and second metal sheets to expand the first and second metal sheets in the unbonded region. The metal sheets may be aluminum sheets. The sheets may be positioned in a die having a plurality of cavities such that when the pressurized gas is introduced the sheets expand into the cavities. The diffusion bonding may be performed by applying pressure, for example, using rollers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/892,861 filed May 13, 2013, now abandoned, the disclosure of which ishereby incorporated in its entirety by reference herein.

BACKGROUND

There are ongoing and increasing demands for weight reduction in motorvehicles. Aluminum being significantly lighter than steel, manymanufacturers are exploring the design of body-in-white (BIW) structuresformed from aluminum. The stiffness required for a given applicationoften defines the minimum gauges usable for the aluminum sheet. Sandwichaluminum structures have the potential to reduce vehicle weightdrastically without compromising durability, safety and NVHperformances. Conventional sandwich aluminum structures may befabricated by stacking and joining several thin aluminum sheets wherethe outer layers are plain and inner layers are formed by creatingplurality of cells using alternating front and rear projections. Due tothe empty space provided by the core cell structure, the resultingsandwich aluminum has significantly lower density than a solid sheetmetal of same overall thickness, resulting in additional weight savings.Although sandwich structures can be formed into desired partconfigurations, the forming pressures can collapse the cores anddramatically reduce the overall thickness of the sandwich. Thus, theoverall strength, crashworthiness and stiffness of the resultingsandwich structure may be adversely affected by the required formingoperations.

Thus, a need exists for beam structures and other load bearingstructures which meet performance requirements and which have lowerweights than existing structures.

SUMMARY

In one aspect of the embodiments described herein, a laminate includestwo pieces of metallic material bonded together at a plurality ofregularly spaced-apart bonded regions.

In another aspect of the embodiments described herein, a laminateincludes two pieces of metallic material bonded together at a pluralityof regions, each region having the same shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a production line structured forfabricating a basic laminate in accordance with the embodimentsdescribed herein.

FIG. 1A is a magnified schematic view of a portion of one the productionline shown in FIG. 1.

FIG. 1B is a detailed view a portion of a laminate constituent sheetafter passing through a printing station.

FIG. 1C shows a detailed schematic view of laminate constituents passingthrough rollers.

FIG. 2 is a cross-sectional view of a section of finished laminatepositioned in a portion of the die prior to injection of pressurized airbetween the constituent layers of the laminate.

FIG. 3 shows a cross-sectional view of a portion of the finishedlaminate of FIG. 2 after injection of pressurized air between theconstituent layers of the laminate.

FIG. 4A is a magnified view of a portion of the view shown in FIG. 3,showing an exemplary die cavity and laminate constituents positionedtherein after injection of pressurized air.

FIG. 4B is a cross-sectional view of a portion of an exemplary dieincorporating multiple die cavities as shown in FIG. 4A.

FIG. 5 shows a perspective view of one embodiment of a part formed froma piece of expanded laminate.

FIG. 6 shows a graphical representation of the relative crush strengthof a laminate in accordance with one of the embodiments describedherein.

FIG. 7 shows a graphical representation of the relative impactresistance of a laminate in accordance with one of the embodimentsdescribed herein.

FIG. 8A shows a schematic view of a part formed from a piece of expandedlaminate in accordance with one of the embodiments described herein,attached to an interior of a vehicle hood as a hood reinforcement.

FIG. 8B shows a schematic view of a part formed from a piece of expandedlaminate in accordance with one of the embodiments described herein,attached to an interior of a vehicle hood as a hood reinforcement.

FIG. 9A is a perspective view of two pieces of expanded finishedlaminate after forming into shapes suitable for use in fabricating anenergy-absorbing crush can.

FIG. 9B shows a cross-sectional end view of the can of FIG. 9A.

FIG. 9C shows a detailed cross-sectional end view of a portion of thecan shown in FIG. 9B.

FIG. 10 is a schematic view of a deposition pattern in accordance withone particular embodiment.

FIG. 11 shows a schematic view of a deposition pattern in accordancewith another particular embodiment.

FIG. 12 shows a schematic view of a deposition pattern in accordancewith another particular embodiment.

FIG. 13 shows a schematic view of a deposition pattern in accordancewith another particular embodiment.

FIG. 14 shows a schematic view of a deposition pattern in accordancewith another particular embodiment.

FIG. 15 shows a schematic view of a deposition pattern in accordancewith another particular embodiment.

FIG. 16 is a detailed cross-sectional view of a portion of a testspecimen formed from an exemplary embodiment of the finished laminate.

FIG. 17 is a schematic view of a test arrangement for testing the sampleshown in FIG. 16.

FIG. 18 is a schematic view of a test arrangement for bend testing of anexemplary sample of the finished laminate.

DETAILED DESCRIPTION

In embodiments described herein, a basic laminate structure usable for avariety of vehicle structural components is formed from diffusion-bondedsheets of aluminum alloy. This laminate structure is stiffer andstronger than both solid aluminum sheet structures and conventionalsandwich aluminum structures, and may be formed into desiredconfigurations using conventional method. The basic laminate produced bythe process described below can be used to form a wide variety ofstructures, including finished vehicle components and braces orreinforcements attachable to other vehicle components for use instrengthening and/or stiffening these other components.

As used herein, the terms “basic laminate” and “finished laminate” referto a laminate comprising two pieces or strips of material bondedtogether as described herein, and prior to any forming or expansionoperations as described below.

In embodiments of the bonding process described herein, the componentsusable for forming the basic laminate comprise two pieces, strips orsheets of one or more suitable aluminum alloys. Either of the laminateconstituents may be formed from any aluminum alloy in the 1000, 2000,3000, 4000, 5000, 6000 or 7000 series.

FIG. 1 is a schematic view of a production line 900 structured forfabricating a basic laminate in accordance with at least one of theembodiments described herein. Line 900 is structured to efficientlyfabricate a quantity of basic laminate in a continuous process, bydiffusion bonding a pair of continuous sheets 902 and 904 of aluminumalloy to each other. In a particular embodiment of the process, each ofthe laminate constituents 902 and 904 has a substantially uniformthickness (within thickness manufacturing tolerances for the componentsheet) within a range of 0.4 mm-1.5 mm inclusive. Sheets 902 and 904 arefed by associated uncoilers (906 and 908, respectively) from rolls ofaluminum sheet. The sheets are then passed through a flattener station910 and a scratch brushing station 912 to aid in removing contaminantsfrom the inner surfaces 902 a and 904 a of the sheets along which thediffusion bonds will be formed, and to roughen the surfaces to promoteformation of the bonds.

In the next step of the process, a spreadable composition of relativelyhigh temperature-resistant material (for example, graphite, aluminumoxide, or another suitable material) is selectively deposited onto atleast one of the surfaces 902 a or 904 a of laminate components 902 and904, respectively, to form a covered portion 801 of the component. Ahigh temperature-resistant material is selected that will not chemicallyreact with the surfaces to be bonded. The high temperature-resistantmaterial is deposited (using a known technique) in a pattern that leavesexposed or uncovered the remaining portions of the surface(s). Thediffusion bonds between the sheets 902 and 904 will be formed at theseexposed surface portions. In the embodiment shown in FIG. 1, the hightemperature-resistant material is deposited on bonding surface 902 a ofsheet 902. In one embodiment, a known silkscreen or screen printingprocess is used to apply the high temperature-resistant material to theselected surface at a printing station 914. However, any suitableprocess may be used.

FIG. 1B is a detailed view a portion of sheet 902 after passing throughthe printing station 914. This view shows one embodiment of a pattern ofdeposition of the high temperature-resistant material covering sheetportion 801 on bonding surface 902 a.

In the embodiment shown in FIG. 1B, discrete circular exposed surface(i.e., exposed metal) portions 802 are separated or spaced apart bycovered portion(s) 801. In addition, as shown in FIG. 1B, portions 810of the laminate constituents 902 and 904 are left uncovered along theedges thereof (except where air flow passages 805 are to be formed asdescribed below). These uncovered portions of the laminate componentsare later bonded to associated uncovered portions of sheet 904 toprevent escape of pressurized air from between the bonded laminatesheets 902 and 904, and to form flanges 934 and 936 bonded to associatededges of sheet 904 and which can be riveted, welded or otherwisesuitably attached to another piece of laminate or to another vehiclecomponent.

In addition, high temperature-resistant material is also deposited suchthat the covered portion 801 of the laminate constituent includes one ormore paths 805 connecting one or more associated edges of the laminatewith the other covered regions of the laminate constituent 902. Afterapplication of bonding pressure, these covered paths later form unbondedair paths or passages between bonded portions of the laminate sheets 902and 904. Thus, the coated portions 801 and 810 of the surface 902 a areinterconnected in a continuous pattern extending from the edges of thelaminate constituent sheet 902 into and through the central portions ofthe sheet.

Although FIG. 1B shows one embodiment of particular patterns and shapesof covered portions 801 and exposed portions 802, any other desiredpattern or arrangement of covered portions and exposed portions may beemployed, depending on the requirements of a particular application. Forexample, the high temperature-resistant material can be applied ordeposited on the bonding surfaces of the laminate constituent sheets 902and 904 in shapes of ellipses, rectangles, circular or serpentinestructures and/or any of numerous other shapes and/or any combinationsthereof. In a particular embodiment, the exposed surface portions 802are generally circular in shape as shown in FIG. 1B. However, theexposed surface portions and the adjacent covered portions 801 of thesurfaces may have any shape or shapes (for example, ellipses,rectangles, serpentine or other shapes) that can be applied to thelaminate component surface(s) using a suitable process. The appliedpattern of unbonded material on sheet 902 forms, when bonded in a laterstep to sheet 904, a network of air flow channels extending between thelaminate constituent sheets 902 and 904, from the edges of the finishedlaminate and through the central portions of the laminate. Thesepassages permit a flow of pressurized air between the laminate sheets902 and 904 for inflating or expanding the laminate structure, asdescribed below.

FIGS. 10-15 show embodiments of deposition patterns hightemperature-resistant material deposited on a bonding surface of atleast one of sheets 902 and 904. In each of the patterns shown in FIGS.10-15, the individual exposed regions of metal surface have the sameshapes and overall dimensions, within the limits of fabrication andbonding process tolerances. However, in other pattern embodiments, theindividual exposed regions within each pattern may have differing shapesand/or dimensions, according to the requirements of a particularapplication.

As used herein, the term “regularly spaced apart” as applied to theexposed bonding regions formed on a bonding surface of a laminateconstituent is understood to mean that the centers of all regions lyingon a straight line extending from the center of any given region arespaced apart the same distance from each other.

In general the parameters of a given deposition pattern (such as theshapes of the exposed regions of high temperature-resistant material,the dimensions of the exposed regions, the spacing between the exposedregions, the distances of the exposed regions from the edges of theconstituent sheet on which they are formed, and any other parameterspertinent to the particular pattern of deposition) will be selected soas to impart a strength and/or stiffness within a desired range to apart formed from the finished laminate.

Generally the stiffness and strength of a piece of finished pressureroll-bonded laminate 294 will depend on the ratio of the total unbondedarea to the total bonded area of the piece. Typically, a relativelyhigher proportion of unbonded area will provide a relatively higherstrength and stiffness. Thus, the ranges of strength and stiffness maybe controlled or “tuned” by modifying the unbonded/bonded area ratio. Tomaximize the strength and stiffness, it is generally desirable tominimize the net or total bonded area. However, the total bonded areashould also be sufficiently large to hold the laminate constituentstogether along the bonded portions of the constituents during expansionof the laminate.

The practical maximum value of the unbonded/bonded area ratio for agiven deposition pattern may be determined experimentally by adjustingone or more of the various parameters of the deposition pattern,including the dimensions of the deposited regions of hightemperature-resistant material, the spacing between the regions, thedistances of the regions from the edges of the constituent sheet onwhich they are deposited, and any other parameters pertinent to theparticular pattern of deposition.

In particular embodiments, where the bonded regions in a finishedlaminate have the same shapes and overall dimensions, exposed regions ofthe laminate constituents are configured such that the maximum spacingbetween adjacent bonded regions of the finished laminate will be equalto a maximum overall dimension of an exemplary bonded region.

FIG. 10 shows a schematic view of a deposition pattern in accordancewith one particular embodiment. In FIG. 10, the hightemperature-resistant material is deposited on the surface of at leastone of the laminate constituents so as to leave exposed an array ofdarkened circular areas A1 as shown. Thus, the hightemperature-resistant material resides in the regions surrounding thecircular areas A1. The array includes multiple rows and columns ofcircular exposed areas, which form, when the laminate constituents arebonded together, corresponding circular bonded regions holding togetherthe laminate constituents. The rows include multiple rows R1 of circularareas A1 and multiple rows R2 of circular areas A2. All areas A1 may beequal to each other, and all areas A2 may be equal to each other, withinthe limitations of the deposition process. Also, all the areas A1 and A2may have the same values, within the limitations of the depositionprocess.

The centers of areas A1 along a given row R1 are collinear, and thecenters of areas A2 along a given row R2 are collinear. Rows R1 and R2are arranged in alternating fashion along a direction S extendingperpendicular to a line connecting the centers of the circular areaspositioned along a given row. Each row R1 is spaced apart a distance Y1from an adjacent row R2, and a distance 2Y1 from another adjacent rowR1. Adjacent centers of areas A1 along any given row R1 are spaced aparta distance 2X1, and adjacent centers of areas A2 along any given row R2are spaced apart a distance 2X1. The center of each area A2 in a row R2is spaced apart from a center of an area A1 in an adjacent row R1 adistance D, where D is given by the relation:D=(X1² +Y1²)^(1/2)

Although the embodiment of FIG. 10 shows an arrangement of six rows R2interspersed with seven rows R1, a lesser or greater number of rows,columns, or circular exposed areas may be incorporated into a givenarrangement of exposed areas, depending on the requirements of aparticular application.

FIG. 11 shows a schematic view of a deposition pattern in accordancewith another particular embodiment. In FIG. 11, the hightemperature-resistant material is deposited on the surface of thelaminate constituent so as to leave an array of elliptical exposed areasA3 arranged in columns CC1 and rows RR1 which form, when the laminateconstituents are bonded together, corresponding elliptical bondedregions holding together the laminate constituents. Thus, the hightemperature-resistant material resides in the regions surrounding theelliptical exposed areas A3. Semi-major axes of the ellipses in eachcolumn CC1 are arranged collinearly, and semi-minor axes of the ellipsesin each row RR1 are arranged collinearly. The lines along which thesemi-major axes of the ellipses in adjacent columns CC1 extend arespaced apart a distance X2, and the lines along which the semi-minoraxes of the ellipses in adjacent rows RR1 extend are spaced apart adistance Y2. Also, all the areas A3 may have the same values, within thelimitations of the deposition process. Although the embodiment of FIG.11 shows an arrangement of four rows RR1 and ten columns CC1 ofelliptical exposed areas, a lesser or greater number of rows, columns,or elliptical exposed areas may be incorporated into a given arrangementof exposed areas, depending on the requirements of a particularapplication.

FIG. 12 shows a schematic view of a deposition pattern in accordancewith another particular embodiment. In FIG. 12, the hightemperature-resistant material is deposited on the surface of thelaminate constituent so as to leave an array of exposed rectangularareas A4 arranged in columns CC2 and rows RR2 which form, when thelaminate constituents are bonded together, corresponding rectangularbonded regions holding together the laminate constituents. Thus, thehigh temperature-resistant material resides in the regions surroundingthe rectangular exposed areas A4. Centers of the rectangular areas ineach column CC2 are arranged collinearly, and centers of the rectangularareas in each row RR2 are arranged collinearly. The lines along whichthe centers of the rectangular areas in adjacent columns CC2 extend arespaced apart a distance X3, and the lines along which the centers of therectangular areas in adjacent rows RR2 extend are spaced apart adistance Y3. Also, all the areas A4 may have the same values, within thelimitations of the deposition process. Although the embodiment of FIG.12 shows an arrangement of three rows RR2 and ten columns CC2 ofrectangular exposed areas, a lesser or greater number of rows, columns,or rectangular exposed areas may be incorporated into a givenarrangement of exposed areas, depending on the requirements of aparticular application.

FIG. 13 shows a schematic view of a deposition pattern in accordancewith another particular embodiment. In FIG. 13, the hightemperature-resistant material is deposited on the surface of thelaminate constituent so as to leave an array of exposed cross-shapedareas A5 arranged in columns CC3 and rows RR3 which form, when thelaminate constituents are bonded together, corresponding cross-shapedbonded regions holding together the laminate constituents. Thus, thehigh temperature-resistant material resides in the regions surroundingthe cross-shaped exposed areas A5. Centers of the cross-shaped areas ineach column CC3 are arranged collinearly, and centers of thecross-shaped areas in each row RR3 are arranged collinearly. The linesalong which the centers of the cross-shaped areas in adjacent columnsCC3 extend are spaced apart a distance X4, and the lines along which thecenters of the cross-shaped areas in adjacent rows RR3 extend are spacedapart a distance Y4. Also, all the areas A5 may have the same values,within the limitations of the deposition process. Although theembodiment of FIG. 13 shows an arrangement of four rows RR3 and sevencolumns CC3 of cross-shaped exposed areas, a lesser or greater number ofrows, columns, or cross-shaped exposed areas may be incorporated into agiven arrangement of exposed areas, depending on the requirements of aparticular application.

FIG. 14 shows a schematic view of a deposition pattern in accordancewith another particular embodiment. In FIG. 14, the hightemperature-resistant material is deposited on the surface of thelaminate constituent so as to leave an array of exposed “S”-shaped areasA6 arranged in columns CC4 and rows RR4 which form, when the laminateconstituents are bonded together, corresponding “S”-shaped bondedregions holding together the laminate constituents. Thus, the hightemperature-resistant material resides in the regions surrounding the“S”-shaped exposed areas A6. In the embodiment shown in FIG. 14, all ofthe “S”-shaped areas A6 have the same size, within the limitations ofthe deposition process. An envelope size of a representative “S”-shapedareas is a rectangular area Z1 defined by the maximum dimension S1 ofthe “S”-shape in a first direction (in FIG. 14, the direction in whichrows RR4 extend) and the maximum dimension S2 of the “S”-shape in asecond direction extending perpendicular to the first direction.

Centers of the rectangular areas Z1 of the S″-shape areas in each columnCC4 are arranged collinearly, and centers of the rectangular areas Z1 ofthe S″-shape areas in each row RR4 are arranged collinearly. The linesalong which the centers of the rectangular areas Z1 in adjacent columnsCC4 extend are spaced apart a distance X5, and the lines along which thecenters of the cross-shaped areas in adjacent rows RR4 extend are spacedapart a distance Y5. Although the embodiment of FIG. 14 shows anarrangement of five rows RR4 and twelve columns CC4 of S″-shaped exposedareas, a lesser or greater number of rows, columns, or S″-shaped exposedareas may be incorporated into a given arrangement of exposed areas,depending on the requirements of a particular application.

FIG. 15 shows a schematic view of a deposition pattern in accordancewith one particular embodiment. In FIG. 15, the hightemperature-resistant material is deposited on the surface of thelaminate constituent so as to leave exposed an array of square-shapedareas as shown and which form, when the laminate constituents are bondedtogether, corresponding square-shaped bonded regions holding togetherthe laminate constituents. Thus, the high temperature-resistant materialresides in the regions surrounding the square-shaped areas A7 and A8.The array includes multiple rows and columns of square-shaped exposedareas. The rows include multiple rows RR5 of square-shaped areas A7 andmultiple rows RR6 of square-shaped areas A8. All areas A7 may be equalto each other, and all areas A8 may be equal to each other, within thelimitations of the deposition process. Also, all the areas A7 and A8 mayhave the same values, within the limitations of the deposition process.

The centers of areas A7 along a given row RR5 are collinear, and thecenters of areas A8 along a given row RR6 are collinear. Rows RR5 andRR6 are arranged in alternating fashion along a direction S extendingperpendicular to a line connecting the centers of the square-shapedareas positioned along a given row. Each row RR5 is spaced apart adistance Y6 from an adjacent row RR6, and a distance 2Y6 from anotheradjacent row RR5. Each row RR6 is spaced apart a distance 2Y6 from anadjacent row RR6. Adjacent centers of areas A7 along any given row RR5are spaced apart a distance 2X6, and adjacent centers of areas A8 alongany given row RR6 are spaced apart a distance 2X6. The center of eacharea A8 in a row RR6 is spaced apart from a center of an area A7 in anadjacent row RR5 a distance D6, where D6 is given by the relation:D6=(X6² +Y6²)^(1/2)

Although the embodiment of FIG. 15 shows a particular number of rows RR6interspersed with a particular number of rows RR5, a lesser or greaternumber of rows, columns, or square-shaped exposed areas may beincorporated into a given arrangement of exposed areas, depending on therequirements of a particular application.

In certain instances, application of pressure to the laminate components902, 904 at elevated temperatures may facilitate diffusion bonding ofthe abutting laminate component surfaces 902 a and 904 a to each other,thereby reducing the bonding pressure required to achieve the desiredbond strength. Thus, it may be desired to pre-heat the components 902and 904 prior to application of bonding pressure. For this purpose, asseen in FIG. 1, line 900 may incorporate a heating station 920 usablefor pre-heating the laminate components 902 and 904 after the screeningoperation and prior to application of pressure.

In the embodiment shown in FIG. 1, heating station 920 is a continuousfurnace configured for heating the laminate constituents to a desiredpre-heat temperature prior to application of pressure. However, anyother suitable type of furnace or heat source may be used. In certainembodiments, the sheets 902 and 904 are heated to a temperature withinthe range of 300-580° C., depending on the alloy or alloys being bonded,the desired bond strength, and other pertinent factors. In cases whereit is not desired or needed to heat the laminate components prior toapplication of pressure, the furnace or heating station may be shutdown.

In particular embodiments of the laminate fabrication process, a knownflux material (for example, a potassium aluminum fluoride flux)(notshown) may be applied to one or more of the abutting surfaces 902 a, 904a of the laminate components 902 and 904 prior to application ofpressure, to loosen and/or remove oxides from the abutting surfaces. Byaiding in oxide removal, application of the flux may enhance diffusionrates in the laminate components, thereby enabling a further reductionin the pressure required to achieve a desired bond strength.

Line 900 is structured to apply pressure (and, optionally, elevatedtemperatures) required to diffusion bond exposed portions surfaces 902 aand 904 a together. For this purpose, a bonding station 922 is providedclose to the area where the pre-heated laminate components 902 and 904leave the heating station 920. In the line embodiment shown in FIG. 1,the required bonding pressure is applied by a suitably configured set ofrollers (generally designated 922 a) between which the laminatecomponents 902 and 904 are passed, in a known manner. However, othermethods of applying the required bonding pressure may also be used. FIG.1C shows a detailed schematic view of laminate constituents 902 and 904passing through the rollers. The speed at which laminate constituents902 and 904 are fed through the rollers 922 a, the pressure applied bythe rollers, and other line operating parameters are adjustable to applysufficient pressure to achieve a diffusion bond of a desired strengthbetween laminate constituents at a given temperature, in a manner knownin the art. Thus, the process line 900 may be adapted to bond togethersheets formed from any of a variety of alloys.

During application of bonding pressure to the abutting sheets 902 and904, the constituent layers of the laminate are diffusion bonded to eachother at the exposed portions 802 and 810 of the surfaces, while thecoated portions 801 of the surface 902 a remain unbonded to surface 904a of sheet 904. The elevated temperature of the exposed metal regionsfacilitates the formation under pressure of diffusion bonds between thevarious exposed metal surfaces. As stated previously, these bonded edgeportions 810 of the finished laminate form flanges 934 and 936 which canbe riveted, welded or otherwise suitably attached to another piece oflaminate or to another vehicle component.

FIG. 1B shows a magnified schematic view of laminate constituents 902and 904 passing through rollers 922 a. Also, after passing betweenrollers 922 a, the thickness of the finished laminate 924 may be up to60% less than the combined thicknesses of the sheets 902 and 904 priorto application of the bonding pressure. In addition, compression of thelaminate constituent sheets 902 and 904 during application of bondingpressure produces a longitudinal (in the material feed direction) andlateral (transverse to the material feed direction) “spreading” of thelaminate sheets which results in an increase in the overall size of thepattern defining the unbonded regions of the laminate. In particularembodiments, the total thickness of the finished laminate 924 leavingthe pressure-application station of the line 900 is in the range of 0.8millimeter (mm) to 3.0 mm inclusive.

The finished basic laminate 924 may be wound onto a take-up roller orrecoiler 930, as shown in FIG. 1. A section of finished laminate 924 maynow be cut from the roll for forming, cutting or shaping into a desiredpart shape prior to the inflation or expansion step. A section of thefinished laminate may be formed using known techniques into any of avariety of complex shapes as desired. Even deep drawn pats can be formedfrom the laminate.

In the next step, the formed section of finished laminate is secured ina suitably shaped die 710 in which pressurized air is introduced betweenthe laminate constituent layers 902 and 904 to expand or inflate thefinished laminate. The die 710 is configured to enable high-pressure airto be injected from the edges of the finished laminate piece betweenconstituent layers 902 and 904 via passage(s) formed along the unbondededge portions 805 of the laminate layers.

FIG. 2 is a cross-sectional view of a section of finished laminate 924positioned in a portion of the die 710 prior to injection of pressurizedair between the constituent layers 902 and 904. In the embodiment shown,die 710 has a first portion 712 and a second portion 714. first portion712 has a plurality of cavities 712 a formed along interior surfacesthereof, and dies second portion 714 has a plurality of cavities 714 aformed along interior surfaces thereof, each cavity 714 a beingpositioned opposite an associated one of cavities 712 a. These cavities712 a, 714 a are configured to permit expansion of the unbonded portions902 x, 904 x of the laminate constituents 902 and 904 therein duringapplication of the pressurized air. That is, the die cavities 712 a, 714a are positioned so as to correspond to or overlie the unbonded portions902 x, 904 x of the final laminate piece 924 when the laminate piece ispositioned in the die. Similarly, the portions of the die surroundingthe cavities 712 a, 714 a are configured so as to correspond to oroverlie the bonded portions 924 w of the laminate piece. These portionsof the die act to brace or support the bonded portions 924 w of thelaminate against the forces exerted by the pressurized air, therebyminimizing the stresses on the bonded portions.

FIGS. 3 and 4A show the cross-sectional views of portions of FIG. 2after injection of pressurized air between the constituent layers 902and 904. FIG. 4B is a cross-sectional view of a portion of an exemplarydie incorporating cavities as shown in FIG. 4A, and prior to placementof laminate constituents 902 and 904 therein. As seen from FIGS. 2-4B,as the pressurized air P is introduced into passage(s) 805, the airflows between the laminate constituents 902 and 904 along interconnectedpassages defined by the unbonded regions of the laminate. Theinterconnected unbonded portions of the expanded laminate residingbetween the bonded portions thus combine to form a continuous unbondedregion extending around the bonded regions. This airflow causes theunbonded portions of the laminate along both constituents 902 and 904expand into the die cavities 712 a, 714 a along each side of the die. Inthis manner, the laminate piece 924 is inflated or expanded. Theexpansion process increases the overall effective thickness of thefinished laminate piece. In particular embodiments, the final overallpart thickness D is within the range 4.0 mm-8.0 mm.

The air pressure needed for inflating or expanding the laminatestructure after forming will depend on such factors as the thickness(es)of the laminate components 902 and 904, the geometry of the formed part,the yield strength of the alloy(s) used for the laminate, and otherpertinent factors. Typical inflation pressures may be in the range 20MPa to 200 MPa.

FIG. 5 shows one embodiment of a finished (in an expanded orpost-inflation state) part 600 structured for use as a reinforcing platefor another portion (not shown) of the vehicle. Part 600 may be securedto other portion of portions of the vehicle along flanges 934 and 936.FIGS. 8A and 8B show a part 600 a constructed in a manner similar tothat of part 600 attached to an interior of a vehicle hood 1200 as areinforcing member.

In another embodiment (shown in FIGS. 9A-9C), one or more pieces 610 and620 of finished laminate 924 are formed so as to able their combinationinto a box-shaped crush can 630 after the pieces have been inflated.FIG. 9A is a perspective view of two pieces 610 and 620 of expanded orinflated finished laminate after forming into shapes suitable for use infabricating an energy-absorbing crush can 630. Part 620 is bent prior toexpansion to form a pair of right angle bends 622 and 624, while part610 is formed from a flat expanded piece of finished laminate. FIG. 9Bshows a cross-sectional end view of the can of FIG. 9A. FIG. 9C shows adetailed cross-sectional end view of a portion of the can shown in FIG.9B. This view clearly shows the bonded portions 610 a and 620 a of theparts 610 and 620, and also the unbonded and expanded portions 610 b and620 b of the parts.

Novel applications of embodiments of the pressure roll-bonded aluminumlaminate described herein include BIW and closure structures currentlyfabricated using solid aluminum sheets. Other collision-criticalstructural applications for the expanded laminate include light-weightbumpers, B-pillars, vehicle floor cross members, roof bows, and numerousother applications. Other possible applications include hoodreinforcements, dash reinforcements, floor reinforcements, and any otherapplications where stiffness is required without a high weight penalty.

EXAMPLES

Referring to FIG. 16, to evaluate the relative stiffness and strength ofthe finished laminate with respect to alternative aluminum and steelstructures, a finished test laminate was used to construct crush-can1000 having a structure similar to that shown in FIGS. 9A-9C constructedwith two sheets of 0.625 mm thick aluminum alloy 5754 as the laminateconstituents 902 and 904 bonded at a plurality of spaced apart circularbonding areas, using the method previously described.Similarly-structured cans were fabricated from 0.8 mm thick low carbonmild steel and 1.8 mm thick aluminum alloy 5754.

The diameters of the circular bonding regions at non-bonding surfaces ofthe constituent sheets (i.e., at surfaces of the sheets opposite thebonding surfaces 902 a and 904 a) were about 10.125 mm. The diameters ofthe circular bonding regions at bonding surfaces of the constituentsheets were about 8 mm. The centers of the circular bonding regions werespaced about 13.6 mm from each other along the extent of the laminate.The spacing between the unbonded portions of the bonding surfaces 902 aand 904 a of the finished laminate was about 3.225 mm. During loading,one end 1000 a of the crush can was constrained by a stationary rigidwall 1010, while an opposite end 1000 b of the can 1000 was loaded usinga rigid wall 1012 moving at 10 m/s (about 22.4 MPH).

The energy expended to crush each test can sample a given distance wasmeasured, using the test arrangement shown in FIG. 17. The test resultsare shown in FIG. 6. It is seen from FIG. 6 that the energies requiredto crush each sample a given amount were comparable to within a range of+/−10%. However, while each sample occupied the same overall volume orenvelope size, the weight of the steel sample was 2.24 lb, the weight ofthe single aluminum sheet was 1.7 lb, and the weight of the laminatesample was 1.12 lb. Thus, for samples of the same size, the laminateprovided a weight savings of 30% over the single, thicker aluminum sheetand a weight savings of 50% over the steel sample.

To evaluate the relative impact resistance of the finished laminate withrespect to alternative aluminum and steel structures, a section offinished test laminate was constructed with two sheets of 0.625 mm thickaluminum alloy 5754 as the laminate constituents 902 and 904 bonded at aplurality of spaced apart circular bonding areas, using the methodpreviously described. Test samples were also fabricated from 0.8 mmthick low carbon mild steel and 1.8 mm thick aluminum alloy 5754. Eachtest sample was 300 mm in length. A 3-point bend test was conducted oneach sample using the test arrangement shown in FIG. 18. The test sampleis generically labeled 1300 in FIG. 18. The arrangement shown in FIG. 18is designed to induce bending of the test specimen responsive totransverse impact loading. The arrangement has spaced-apart 30 mmdiameter rigid supports 1302 and 1304. A 50 mm diameter impactor applieda force F midway between the supports.

The test results are shown in FIG. 7. It is seen from FIG. 7 that theforces generated by impact of the impactor on the test specimens werecomparable to within a range of +/−10%. However, the weight of the steelsample was 3.5 lb, the weight of the single aluminum sheet was 1.8 lb,and the weight of the laminate sample was 1.12 lb. Thus, for samples ofthe comparable resistance to bending forces, the laminate provided aweight savings of 30% over the single, thicker aluminum sheet and aweight savings of 68% over the steel sample.

It will be understood that the foregoing descriptions of the variousembodiments are for illustrative purposes only. As such, the variousstructural and operational features herein disclosed are susceptible toa number of modifications, none of which departs from the scope of theappended claims.

What is claimed is:
 1. A method comprising: selectively applying amaterial to a surface of a first metal sheet to form an uncoveredportion having an array of exposed areas; diffusion bonding the firstmetal sheet to a second metal sheet at the uncovered portion to formdiscrete, spaced apart bonded regions that are formed as crosses alongthe array, wherein the diffusion bonding step includes applying pressureto the first and second metal sheets via a pair of rollers; andintroducing pressurized gas between the first and second metal sheets toexpand the first and second metal sheets in a single unbonded regionthat completely surrounds an outer perimeter of each bonded regionwithin the array.
 2. The method of claim 1, wherein the first and secondmetals sheets are aluminum alloy sheets.
 3. The method of claim 1,wherein the material includes graphite or aluminum oxide.
 4. The methodof claim 1, wherein the bonded regions and the unbonded region have asame initial thickness and the unbonded region has a finished thicknessthat is greater than the initial thickness after the pressurized gas isintroduced.
 5. The method of claim 1, wherein the first metal sheetincludes a covered portion having an edge portion such that during thediffusion bonding step an unbonded edge portion is formed.
 6. The methodof claim 5, wherein the pressurized gas is introduced at the unbondededge portion.
 7. A method comprising: selectively applying a coating toa surface of a first aluminum sheet to form a coated portion anduncoated portions having an array of exposed areas; diffusion bondingthe first aluminum sheet to a second aluminum sheet at only the array ofexposed areas to form bonded regions along the array and a singleunbonded region that completely surrounds an outer perimeter of eachbonded region within the array, wherein the array includes discrete,spaced apart bonded regions formed as crosses, wherein the first andsecond aluminum sheets are heated to a temperature of 300° C. to 525° C.prior to applying pressure during the diffusion bonding step, andwherein the diffusion bonding step includes applying pressure to thefirst and second aluminum sheets via a pair of rollers while thetemperature of the first and second aluminum sheets is between 300° C.to 525° C.; and introducing pressurized gas between the first and secondaluminum sheets to expand the first and second aluminum sheets in theunbonded region.
 8. The method of claim 7, wherein the bonded regionsand the unbonded region have a same initial thickness and the unbondedregion has a finished thickness that is greater than the initialthickness after the pressurized gas is introduced.
 9. The method ofclaim 7 further comprising: positioning the first and second aluminumsheets into a die including a first portion having a first cavity and asecond portion having a second cavity, the first and second aluminumsheets positioned such that the first and second cavities overlie theunbonded region; and introducing pressurized gas between the first andsecond aluminum sheets to expand the first aluminum sheet into the firstcavity and the second aluminum sheet into the second cavity.
 10. Themethod of claim 7, wherein the uncoated portions include edges of thefirst and second aluminum sheets except for an edge passage portion ofthe coated portion that during the diffusion bonding step forms anunbonded edge passage portion.
 11. The method of claim 10, wherein thepressurized gas is introduced at the unbonded edge passage portion. 12.The method of claim 1, wherein the bonded regions are formed along thearray such that the bonded regions are arranged in a series of columnsand rows.
 13. The method of claim 1, wherein the ratio of an area of thesingle unbonded region relative to an area of the bonded regions withinthe array is greater than one to one.
 14. The method of claim 7, whereinthe bonded regions are formed along the array such that the bondedregions are arranged in a series of columns and rows.
 15. The method ofclaim 7, wherein the ratio of an area of the single unbonded regionrelative to an area of the bonded regions within the array is greaterthan one to one.
 16. The method of claim 1, further comprising heatingthe first and second metal sheets to a temperature of 300° C. to 525° C.prior to applying pressure during the diffusion bonding step, andwherein the diffusion bonding step includes applying pressure to thefirst and second metal sheets via the pair of rollers while thetemperature of the first and second metal sheets is between 300° C. to525° C.
 17. A method comprising: selectively applying a material to asurface of a first metal sheet to form an uncovered portion having anarray of exposed areas; diffusion bonding the first metal sheet to asecond metal sheet at the uncovered portion to form discrete, spacedapart bonded regions that are formed as S-shapes along the array,wherein the diffusion bonding step includes applying pressure to thefirst and second metal sheets via a pair of rollers; and introducingpressurized gas between the first and second metal sheets to expand thefirst and second metal sheets in a single unbonded region thatcompletely surrounds an outer perimeter of each bonded region within thearray.